I suspect that if they tried that it would be a huge backlash and the republicans would regain the house in 2008 as well as retain the White House. People want them to work on their behalf, not do impeachment nonsense.
What about impeachment in the case of the judiciary such as Supreme Court justices who render particularly eggregious decisions? (The recent eminent domain decision in Kelo v City of New London comes to mind.) Would it be good to keep the courts from crossing the line by holding out impeachment as a threat or would that deprive the courts of too much independence? Do you think that incompetent judicial decisions are ground for impeachment in the Judicial sphere? (If I recall correctly, in a Judicial impeachement a judge is NOT automatically removed from office upon conviction. After a judge is convicted another vote is taken to decide the punishment and whether he should be removed and barred from holding future office).